The present invention relates to a photometric measuring device making it possible to automatically study complex solutions with variable "background noise."
More specifically, the invention relates to a device making it possible to sequentially or quasi-continuously determine the concentration of several chemical species contained in the same solution. This device more particularly makes it possible to follow the concentration variations of these chemical species, e.g. during a dilution, a chemical reaction, a solvent-extraction, a reduction in the strength of the concentration of one or more species in the bath, etc. This solution can have a varying turbidity and/or contain parasitic chemical species which may or may not constitute a variable background noise. These parasitic chemical species can, for example, be constituted by the progressive fouling of the walls or cells or photometric measuring vessels by the solutions under investigation which are never perfectly clear or can be the by-products of chemical reactions, e.g. during the following or monitoring of a chemical reaction. This is generally the case with industrial solutions.
At present, numerous types of colorimeters, photometers or spectrophotometers are known for the purpose of measuring the concentration of dissolved chemical species by applying the Beer-Lambert law.
Photometric determinations which several wavelengths are already covered by numerous patents and the corresponding equipment is commercially available.
Differential photometric analysis with two wavelengths is also known, making it possible to eliminate a background noise or turbidity, assumed to be identical at the two measuring wavelengths. For further details on this differential measurement with two wavelengths, reference can be made, for example, to French Pat. No. 2,106,754, filed on Sept. 23rd 1970 by the COMMISSARIAT A L'ENERGIE ATOMIQUE and entitled "Photometric analyser with two wavelengths for the determination of elements in a solution."
Devices also exist which, for remote measurements, give details by means of optical fibres of the concentration of n-1 chemical species by means of n wavelengths. Calculations provide details on the concentration of these n-1 chemical species and the device is described in French Pat. No. 2,317,638, filed on July 9th 1975 by the COMMISSARIAT A L'ENERGIE ATOMIQUE and entitled "Device for analysing the constituents of a solution by photometric measurement."
At present, the various devices for measuring the concentration of a dissolved chemical species use a system of references necessary for the determination of the optical density and therefore the concentration of said chemical species. This reference device uses, for example, a reference solution which only contains the pure chemical species or a "sampling blank" linked with the solvent absorbing power. For a chemical species dissolved in a solvent, the "sampling blank" is generally formed from said pure solvent.
Moreover, the uses of which devices are limited. Thus, in the case of a high light absorption by the chemical species, the optical density or absorption associated with the species can assume a considerable value. The optical density value is dependent not only on the extinction coefficient of the chemical species, but also the concentration in solution of said species. Under these conditions, it is necessary to sample the solution and perform the necessary number of dilutions, this generally being the case for industrial solutions highly concentrated in one or more chemical species.
In order to permit a direct study of the concentration of the different chemical species in solution, i.e. obviating any sampling of the solution which is a nuisance when monitoring a reaction, it is advantageous to be able to carry out the determination of these concentrations by using a device able to work not only on the absorption peaks, but also on the sides and even the toes of the curves of the absorption spectrum plotted beforehand. The presently known devices are not very suitable for determining the optical density on the sides and toes of curves.